Bit field

A bit field is a term used in computer programming to store multiple, logical, neighboring bits, where each of the sets of bits, and single bits can be addressed. A bit field is most commonly used to represent integral types of known, fixed bit-width. A well-known usage of bit-fields is to represent a set of bits, and/or series of bits, known as flags.[citation needed] For example, the first bit in a bit field can be used to determine the state of a particular attribute associated with the bit field.

A bit field is distinguished from a bit array in that the latter is used to store a large set of bits indexed by integers and is often wider than any integral type supported by the language. Bit fields, on the other hand, typically fit within a machine word, and the denotation of bits is independent of their numerical index.

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"A bit field is set up with a structure declaration that labels each field and determines its width."[1] In C and C++ bit fields can be created using unsigned int, signed int, or _Bool (in C99).

You can set, test, and change the bits in the field using a mask, bitwise operators, and the proper membership operator of a struct (. or ->). ORing a value will turn the bits on if they are not already on, and leave them unchanged if they are, e.g. bf.flag |= MASK; To turn a bit off, you can AND its inverse, e.g. bf->flag &= ~MASK; And finally you can toggle a bit (turn it on if it is off and off if it is on) with the XOR operator, e.g. (*bf).flag ^= MASK; To test a bit you can use an AND expression, e.g. (flag_set & MASK) ? true : false;

Having the value of a particular bit can be simply done by left shifting (<<) 1, n amount of times (or, x << n - log2(x) amount of times, where x is a power of 2), where n is the index of the bit you want (the right most bit being the start), e.g. if you want the value of the 4th bit in a binary number, you can do: 1 << 3; which will yield 8, or 2 << 2; etc. The benefits of this become apparent when iterating through a series of bits one at a time in a for loop, or when needing the powers of large numbers to check high bits.

If a language doesn't support bit fields, but supports bit manipulation, you can do something very similar. Since a bit field is just a group of neighboring bits, and so is any other primitive data type, you can substitute the bit field for a primitive, or array of primitives. For example, with a 32 bit integer represented as 32 contiguous bits, you could use it the same way as a bit field with one difference; with a bitfield you can represent a particular bit or set of bits using its named member, and a flag whose value is between 0, and 2 to the nth power, where n is the length of the bits.